Rhomb-dominated quartz CPOs and microfractures of late Variscan granites of the Bavarian Pfahl Zone (W-Bohemian Massif): Implications on the paleostress fields of felsic plutonites.

The Bavarian Pfahl Zone represents a late Variscan, NW-SE striking, transpressive shear zone intruded by large volumes of syn- to post-kinematic felsic plutonites. In this setting, we investigate the relationship between anisotropic plutonic fabrics and the regional tectonic setting for a series of granite intrusions. We combine the analysis of crystallographic preferred orientation (CPO) and shape preferred orientation (SPO) of rock-forming minerals with the orientation of microfractures. Large-area electron backscatter diffraction (EBSD) is used to analyze the CPO and SPO of the main mineral phases. The orientation of open and healed microfracture sets is determined by U-stage measurements. Early healed microfractures recorded by secondary fluid inclusion traces are interpreted to result from hydraulic fracturing in a deviatoric stress field. Samples show up to two orthogonal fracture sets per sample, explained by the orientation of σ2 and σ3 shortly after crystallization. Quartz indicates common, weak, but distinct, rhomb-dominated CPOs for all samples. With respect to the geographic reference frame, three orthogonal rhomb maxima are independent of magmatic flow fabrics, but correlate with the orientation of healed microfracture sets. We interpret these CPOs as the result of stress-induced Dauphiné twinning. Furthermore, we discuss the common spatial relationship of quartz rhomb distributions and healed microfractures on the late- to post-magmatic stress field of felsic plutonites.